In manufacturing a fastener such as a screw or bolt, one end of a section of metal wire or rod is typically axially compressed with a punch or the like with sufficient force to expand the end portion of the wire radially outwardly so as to form a head thereon. The shank is then passed through a thread roller to provide the desired threads.
During axial compression of the wire, the metal tends to randomly expand outwardly, substantially without control. This random expansion occasionally provides a fastener head that is slightly out of round and/or off-center and/or off square with respect to the central axis of the shank which can present problems in future manufacturing processes and sometimes during use.
In many applications it also is desirable to provide a fastener unit having a fastener and a pre-attached washer. The washer is usually inserted on the shank of a fastener or screw blank before the threads are formed on the shank. An example of an apparatus for high speed assembly of a washer to a shank of a fastener blank is illustrated in U.S. Pat. No. 3,750,206 which is assigned to the present assignee and is hereby incorporated by reference.
In that patent, washers and fastener blanks are sequentially advanced for insertion of the shank of the fastener blank through the aperture of the washer. The shank is then threaded for a desired application, which also retains the washer on the shank, and the finished fastener unit is off-loaded for further processing and/or packaging.
Although such an apparatus is quite effective in assembling fastener units, it employs a complex mechanical design that does not positively grip the fastener blank for accurate placement but relies on a guide mechanism and associated components for placement of the fastener shank within the washer. The guide mechanism and associated components are susceptible to misalignment over time which can reduce the efficiency of the apparatus if not monitored and adjusted periodically.
Fastener blank transporting mechanisms have also been developed that grip the head or the underside of the head of the fastener blank, as opposed to the shank, and position the shank for placement within a washer. An example of such a mechanism is illustrated in FIG. 5.
Due to the imprecision in the manufacture of the fastener shanks and heads to form the fastener blanks, however, such existing mechanisms do not perform to desired expectations. This is primarily due to the fact that the mechanism grips the fastener heads or the undersides of the fastener heads which will not enable the shank to be inserted within the washer if the head is slightly eccentric, off-center, or off square with respect to the shank, or if other similar manufacturing defects exist.